The present invention relates to an anisotropically electrocondutive adhesive composition or, more particularly, to an anisotropically electroconductive adhesive composition which serves to electrically connect a pair of terminal electrodes on two circuit boards or, one, on a circuit board and, the other, on an electronic component with simultaneous adhesive bonding of the two circuit boards or the circuit board and electronic component by being interposed therebetween followed by heating under pressure.
It is well known that an insulating adhesive composition compounded with an electrically conductive particulate or fibrous material dispersed therein exhibits anisotropic electroconductivity. Furthermore, it is also well known that the anisotropically electroconductive adhesive compositions of this type are satisfactorily used for connecting integrated circuits, various kinds of display units and other electronic components with a circuit board for driving the electrical component. Various kinds of electrically conductive particulate and fibrous materials are known and used as a component of such an anisotripically electroconductive adhesive composition in the prior art including particles of a metal such as gold, silver, nickel, aluminum, iron and the like as taught in Japanese Patent Publication 58-56996, particles of a metal or plastic having a plating layer of a noble metal and the like on the surface thereof as taught in Japanese Patent Kokai 51-135938, carbon black as taught in Japanese Patent Publication 60-52187, graphite powders as taught in Japanese Patent Publication 61-9342, carbon fibers or ceramics as taught in Japanese Patent Publication 59-64685 and so on.
However, each of these known conductive particulate and fibrous materials has its own disadvantages and problems. For example, metal particles are chemically unstable so that an increase in the contact resistance in the lapse of time is unavoidable eventually leading to disconnection in the circuit during long term use by the oxidation or sulfidization due to contacting with moisture or polluted air penetrating along the boundaries with the insulating adhesive resin enveloping the particles or the interface between the bonded substrate and the adhesive. Particulate materials plated with a noble metal are chemically stable to be freed from the above mentioned drawbacks but they are industrially not fully practical due to the outstanding expensiveness. Carbon blacks, carbon fibers, graphite powders and ceramic powders are also chemically stable. However, it is substantially impossible to establish electric connection between a pair of electrodes through particles of carbon black having an extremely fine particle size as small as 3 nm or even smaller. Carbon fibers cannot be used as a dispersed phase in a conductive adhesive composition to connect circuit boards bearing electrode terminals of 0.6 mm or smaller in size due to the leak by the fibers having a length to bridge between adjacent electrodes despite the sufficiently small diameter of around 5 to 15 .mu.m. Graphite particles cannot be suitably used as a conductive material because of their brittleness as is evidenced by their compression strength as low as 300 to 1000 kg/cm.sup.2 to be eventually crushed or smashed by the influences of heat or compressive force in the course of the bonding works.
The conductive particulate materials prepared by providing a plating layer of a noble metal on the particles of a metallic powder or microspheres of a ceramic, such as inorganic glass, alumina and the like, or a plastic have various disadvantages such as pinholes usually found in the plating layer and crushing of the particles or exfoliation of the plating layer by the mechanical stress which the particles receive in the course of compounding or adhesive bonding works eventually leading to failure in the electric connection or short-circuiting between adjacent terminals.
Ceramic powders, though chemically stable, are also not free from some disadvantages including the usually low electrical conductivity with a specific resistance of as high as 10.sup.-2 ohm.cm or higher and abrasiveness against the surface of the substrate, such as a transparent electrode, bonded with the adhesive to cause cloudiness.
The configuration of the conductive particles used in the prior art is diversified and includes many types such as spherical particles, granular particles, dendritic particles, flaky particles, acicular particles, polyhedral particles, spongy particles, irregularly shaped particles and the like. Among these varieties, however, those having an isotropic configuration are preferred in consideration of the stability in contacting and easiness of screening to obtain uniformity in the particle size distribution and, more particularly, spherical particles are the most suitable in an additional consideration of the relationship between the film thickness and the particle size. Though highly preferable, conductive powders of spherical particles available on the market are limited to the powders of metals such as silver, copper, copper-tin alloys, solder alloys and the like and metal-plated particles of non-conductive substances such as inorganic glass, alumina and plastics including epoxy resins and polystyrene resins. These spherical particles having conductivity are accompanied by chemical and mechanical instability as mentioned before to cause poor electrical connection.
In addition, it is difficult to obtain a conductive adhesive composition containing a metal or ceramic powder as the conductivity-imparting uniformly dispersed phase in a matrix of an insulating adhesive resin due to eventual settling of the powder in the adhesive resin to cause uneven distribution of the particles in the matrix as a consequence of the relatively large specific gravity thereof. In particular, drawbacks of failure in electrical connection or leakage of current sometimes take place, especially, when the arrangement pitch of the electrode terminals is, for example, 0.4 mm or smaller.